The U.S. has a real estate problem. It appears we don't have enough homes to put a roof over everyone's heads.

Say what?

That's the warning from housing economist David Crowe, who projects that the number of homes now being built – on pace toward 591,000 homes and 87,000 rental apartments in 2010 -- isn't enough to keep up with an ever-rising American population. He says that the nation will need to build 16 million new homes over the next decade, or more than twice as many as are coming online now, to keep up with demand. Crowe hints that he's not alone in his concern: "Economists are beginning to sound the warning that today's extremely low levels of new residential production could lead to significant housing shortages."

Now, take this with a grain of salt. David Crowe is the chief economist for the National Association of Home Builders, so part of his job is to inspire fear in the hearts of Congress and government officials across the land to do what's necessary to support residential construction, including propping up the comatose Wall Street market in real estate finance.

As I reported in Rented Spaces recently, vacancy rates have hit record highs in many cities. This week, the blog Calculated Risk pointed out that "the supply of rental units has been surging" (check out this chart to see just how much). Right now the Census counts more than one in every ten rental units as vacant. Add in empty homes for sale and those that are just sitting there collecting tax bills, and it adds up to 14.2 million vacant homes in all. That's a long way toward the magic 16 million number.

And meanwhile, Crowe is understandably alarmed about the record low number of home sales reported this January.

So let's see if we can figure out what in the world Crowe is talking about. Like all such forecasts, his is based on assumptions. For starters, he's betting that most of those Generation Y-ers who are currently holed up with mom and dad are going to decide to strike out on their own when the economy recovers. Then he also figures that the share of renters will continue to grow – with good reason, since more former and would-be homeowners are becoming renters every day. That means a big chunk of the new development will have to be apartment buildings built specifically to be rented. But apartment buildings are big, expensive undertakings that are tough for developers to finance right now.

And then of course a disproportionate amount of of the empty real estate is piling up in states like Michigan and Ohio that residents are fleeing because jobs are impossible to find, or they're on far suburban fringes somewhere where it takes a $50 tank of gas just to get to work. Bottom line: it doesn't count. The trick is to make sure that the areas that are going to see the strongest economic recovery have places for all those workers to live when the jobs finally come.

It's not crazy to think that Crowe and his colleagues are actually right. Back in the Great Depression, real estate development ground to a halt, and just when it started picking up again World War II not only slammed the brakes on construction but drove millions of workers to major cities to be part of the war effort. The result was a real estate shortage so severe that people were living in crates and chicken coops. Brace yourselves - there's a good chance that in tight markets from New York to L.A., economic recovery will bring the biggest real estate squeeze of our lifetimes.